change iterator invalidation avoidance to just move the iterator backward

when something changes, instead of moving forward.  This allows us to 
simplify memset lowering, inserting the memset at the end of the range of 
stuff we're touching instead of at the start.

This, in turn, allows us to make use of the addressing instructions already
used in the function instead of inserting our own.  For example, we now
codegen:

	%tmp41 = getelementptr [8 x i8]* %ref_idx, i32 0, i32 0		; <i8*> [#uses=2]
	call void @llvm.memset.i64( i8* %tmp41, i8 -1, i64 8, i32 1 )

instead of:

	%tmp20 = getelementptr [8 x i8]* %ref_idx, i32 0, i32 7		; <i8*> [#uses=1]
	%ptroffset = getelementptr i8* %tmp20, i64 -7		; <i8*> [#uses=1]
	call void @llvm.memset.i64( i8* %ptroffset, i8 -1, i64 8, i32 1 )

llvm-svn: 48940

lib/Transforms/Scalar/GVN.cpp

@@ -1232,8 +1232,10 @@ void MemsetRanges::addStore(int64_t Start, StoreInst *SI) {
   // See if the range extends the start of the range.  In this case, it couldn't
   // possibly cause it to join the prior range, because otherwise we would have
   // stopped on *it*.
-  if (Start < I->Start)
+  if (Start < I->Start) {
     I->Start = Start;
+    I->StartPtr = SI->getPointerOperand();
+  }
     
   // Now we know that Start <= I->End and Start >= I->Start (so the startpoint
   // is in or right at the end of I), and that End >= I->Start.  Extend I out to
@@ -1345,18 +1347,17 @@ bool GVN::processStore(StoreInst *SI, SmallVectorImpl<Instruction*> &toErase) {
       continue;
     
     // Otherwise, we do want to transform this!  Create a new memset.  We put
-    // the memset right after the first store that we found in this block.  This
-    // ensures that the caller will increment the iterator to the memset before
-    // it deletes all the stores.
-    BasicBlock::iterator InsertPt = SI; ++InsertPt;
+    // the memset right before the first instruction that isn't part of this
+    // memset block.  This ensure that the memset is dominated by any addressing
+    // instruction needed by the start of the block.
+    BasicBlock::iterator InsertPt = BI;
   
     if (MemSetF == 0)
       MemSetF = Intrinsic::getDeclaration(SI->getParent()->getParent()
                                           ->getParent(), Intrinsic::memset_i64);
     
-    // StartPtr may not dominate the starting point.  Instead of using it, base
-    // the destination pointer off the input to the first store in the block.
-    StartPtr = SI->getPointerOperand();
+    // Get the starting pointer of the block.
+    StartPtr = Range.StartPtr;
   
     // Cast the start ptr to be i8* as memset requires.
     const Type *i8Ptr = PointerType::getUnqual(Type::Int8Ty);
@@ -1364,12 +1365,6 @@ bool GVN::processStore(StoreInst *SI, SmallVectorImpl<Instruction*> &toErase) {
       StartPtr = new BitCastInst(StartPtr, i8Ptr, StartPtr->getNameStart(),
                                  InsertPt);
   
-    // Offset the pointer if needed.
-    if (Range.Start)
-      StartPtr = new GetElementPtrInst(StartPtr, ConstantInt::get(Type::Int64Ty,
-                                                                  Range.Start),
-                                       "ptroffset", InsertPt);
-  
     Value *Ops[] = {
       StartPtr, ByteVal,   // Start, value
       ConstantInt::get(Type::Int64Ty, Range.End-Range.Start),  // size
@@ -1695,7 +1690,7 @@ bool GVN::iterateOnFunction(Function &F) {
   
   DominatorTree &DT = getAnalysis<DominatorTree>();   
   
-  SmallVector<Instruction*, 4> toErase;
+  SmallVector<Instruction*, 8> toErase;
   DenseMap<Value*, LoadInst*> lastSeenLoad;
 
   // Top-down walk of the dominator tree
@@ -1713,19 +1708,31 @@ bool GVN::iterateOnFunction(Function &F) {
       currAvail = availableOut[DI->getIDom()->getBlock()];
 
     for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
-         BI != BE; ) {
+         BI != BE;) {
       changed_function |= processInstruction(BI, currAvail,
                                              lastSeenLoad, toErase);
+      if (toErase.empty()) {
+        ++BI;
+        continue;
+      }
       
+      // If we need some instructions deleted, do it now.
       NumGVNInstr += toErase.size();
       
-      // Avoid iterator invalidation
-      ++BI;
+      // Avoid iterator invalidation.
+      bool AtStart = BI == BB->begin();
+      if (!AtStart)
+        --BI;
 
       for (SmallVector<Instruction*, 4>::iterator I = toErase.begin(),
            E = toErase.end(); I != E; ++I)
         (*I)->eraseFromParent();
 
+      if (AtStart)
+        BI = BB->begin();
+      else
+        ++BI;
+      
       toErase.clear();
     }
   }